U.S. Pat. No. 4,975,354 issued Dec. 4, 1990, entitled "Photographic Element Comprising An Ethyleneoxy-Substituted Amino Compound And Process Adapted To Provide High Contrast Development", by Harold I. Machonkin and Donald L. Kerr, describes silver halide photographic elements having incorporated therein a hydrazine compound which functions as a nucleator and an amino compound which functions as an incorporated booster. Such elements provide a highly desirable combination of high photographic speed, very high contrast and excellent dot quality, which renders them very useful in the field of graphic arts. Moreover, since they incorporate the booster in the photographic element, rather than using a developing solution containing a booster, they have the further advantage that they are processable in conventional, low cost, rapid-access developers.
While the invention of U.S. Pat. No. 4,975,354 represents a very important advance in the art, improvement in the photographic elements described therein, particularly with regard to pepper fog and image spread characteristics, is needed.
A photographic system depending on the conjoint action of hydrazine compounds which function as nucleators and amino compounds which function as boosters is an exceedingly complex system. It is influenced by both the composition and concentration of the nucleator and the booster and by many other factors including the pH and composition of the developer and the time and temperature of development. The goals of such a system include the provision of enhanced speed and contrast, together with excellent dot quality and low pepper fog.
The goal of achieving low pepper fog is one which is exceptionally difficult to achieve without sacrificing other desired properties such as speed and contrast. (The term "pepper fog" is commonly utilized in the photographic art, and refers to fog of a type characterized by numerous fine black specks). A particularly important film property is "discrimination", a term which is used to describe the ratio of the extent of shoulder development to pepper fog level. Good discrimination, i.e., full shoulder development with low pepper fog, is necessary to obtain good halftone dot quality.
Image spread in photographic elements of the type described in U.S. Pat. No. 4,975,354 involves infectious imagewise development of unexposed photographic silver halide grains in close proximity to exposed photographic silver halide grains. Like pepper fog, image spread is a detrimental nucleation effect, and means for controlling both pepper fog and image spread are critically needed to improve the performance of these photographic elements.
Herz et al, U.S. Pat. No. 3,220,839 issued Nov. 30, 1965, describes the incorporation of certain isothioureas in photographic emulsions to prevent incubation fog. The photographic elements utilizing these emulsions do not contain a hydrazine compound that functions as a nucleating agent nor an incorporated booster and are not subject to pepper fog formation.
Okutsu et al, U.S. Pat. No. 4,221,857 issued Sep. 9, 1980, describes a high contrast silver halide photographic element containing a hydrazine compound that functions as a nucleator and a polyalkylene oxide compound which serves to minimize formation of drag streaks upon development. The photographic element does not contain an amino compound that functions as an incorporated booster.
Mifune et al, U.S. Pat. No. 4,272,606 issued Jun. 9, 1981, describes a high contrast silver halide photographic element containing a contrast enhancing arylhydrazide and, as an agent which increases sensitivity and contrast, a compound having a thioamido moiety in the molecule thereof. The photographic element does not contain an amino compound that functions as an incorporated booster.
European Patent Application No. 0226184 published Jun. 24, 1987 is concerned primarily with pepper-fog-reducing and image-spread-restraining compounds intended to be incorporated in a developing solution and describes the use of certain isothiourea compounds and certain free mercapto-compounds for this purpose. The photographic elements described do not contain an amino compound that functions as an incorporated booster, but an amino compound is preferably incorporated in the developing solution. While incorporation of the isothiourea compounds and free mercapto-compounds in the photographic element is also disclosed, there is no teaching relating to use of these compounds in a photographic element that contains an incorporated booster. Moreover, the isothiourea compounds described are characterized by features such as the presence of solubilizing groups, which adapt them for most effective use in a developing solution and make them unsuitable for incorporation in a photographic element.
Copending commonly assigned U.S. patent application Ser. No. 599,218 filed Oct. 17, 1990, and issued Jun. 30, 1992, as U.S. Pat. No. 5,126,227, "HIgh Contrast Photographic Elements Containing Ballasted Hydrophobic Isothioureas", by Harold I. Machonkin and Donald L. Kerr describes hydrophobic isothiourea compounds comprising a ballasting group, attached to the sulfur atom, which serves to restrict the mobility of the compound and thereby aid in retaining it in the photographic element during development. When incorporated in a silver halide photographic element containing both a hydrazine compound that functions as a nucleator and an amino compound that functions as a booster, the ballasted hydrophobic isothiourea compound is highly effective in reducing pepper fog and also serves to restrain image spread. However, the ballasted hydrophobic isothiourea compounds exhibit certain shortcomings which have hindered their commercial utilization. Thus, for example, the performance of these compounds is significantly affected by the type of silver halide grains employed, i.e., they don't function equally well with all of the different types of silver halides that are useful in high contrast photographic elements employed in the field of graphic arts. The ballasted hydrophobic isothiourea compounds are believed to release a free mercaptan, which is the active species, during the development step. The free mercaptan then binds to the silver. Because the active species is released by an hydrolysis step, the action of the ballasted hydrophobic isothiourea compounds is very strongly affected by the composition and pH of the developer, so that the choice of developer may be unduly restricted.
The present invention is directed toward the objective of providing novel high contrast silver halide photographic elements which exhibit improved characteristics in regard to control of pepper fog and restraint of image spread, while still retaining excellent characteristics with respect to speed, contrast and full shoulder development.